ICl,swell is the main anion current responsible for RVD. Several aspects of its regulation are still not fully elucidated, but it is known that the protein ICln is one of the main constituent/activators of the channel complex. Many of the interactions of ICln involve members or regulators of the cytoskeleton: this suggests that the cytoskeleton rearrangement that follow an hypotonic challenge could be involved in the translocation of the ICln protein from the cytosol to the cell membrane and in the activation of the ICl,swell. In the present work we focused on the in vivo interaction between ICln and the protein 4.1R, by the use of the FRET technique. Two different isoforms of 4.1R (a long - 4.1Rlong - and a short - 4.1Rsh- isoform, differing for the presence of a 209 aminoacid N-terminal region) were used for the FRET experiments. The strongest FRET signal was measured between ICln and 4.1Rsh. Moreover, when the two proteins are co-expressed, both of them seem to be retained in the cytosol, showing a decreased nuclear and, for 4.1R, membrane localization. The interaction between ICln and 4.1Rlong is weaker, but the co-expression of ICln seems anyway to cause a decrease in the 4.1 membrane localization. Preliminary patch-clamp experiments indicate that the over-expression of the 4.1Rsh isoform in HEK cells leads to an increase of the ICl,swell activation. The emerging picture is that the 4.1R and ICln role in RVD response is far more complex than what initially supposed.